1. Field of the Invention
This invention relates to a catalytic converter for purifying gases using a catalyst with which to purify toxic exhaust gases from an internal combustion engine such as carbon monoxide, hydrocarbons and nitrogen oxides.
2. Description of the Prior Art
Heretofore, exhaust gas converters purifying exhaust gases from internal combustion engines have mostly been provided with a catalyst comprising a noble metal such as platinum, rhodium and palladium. Said catalyst functions to oxidize carbon monoxide and/or hydrocarbons contained in exhaust gases from said engines utilizing residual oxygen in said gases while reducing nitrogen oxides into monocular nitrogen by utilizing said exhaust gases as a reducing agent.
It is necessary in this case to keep said catalyst calcined over 300.degree. C. so as to enhance catalysis. Hence, exhaust gas converters are generally positioned closer to an exhaust manifold, namely where a comparatively higher temperature remains in an exhaust passage, taking into account the co-efficient of the changing temperature of the engine and the target effect of exhaust gas purification.
As mentioned above, according to the prior art the major components of a catalyst comprise a noble metal such as platinum, rhodium and palladium in many instances. Said noble metal is so costly economically that it incurs a large social cost. Further, in view of the increasing consumption in the noble metal, depletion of such a natural resource has become the growing concern.
As indicated above, the catalytic exhaust gas converter utilizing the prior art has generally been so designed as to be positioned closer to an internal combustion engine in an exhaust passage with a view to improving the effect of converting exhaust gases. Or, some heating systems have been disclosed (Japanese Patent Publication Nos. 1993-163935 and 1996-93456) which are designed to electrically maintain a catalyst at a high temperature over 800.degree. C. However, said converters have a disadvantage that such a high temperature affects the catalyst so that its aging is accelerated. Hence, it proves difficult to long maintain the purifying efficiency of the catalyst.
Without the development of a catalytic converter comprising such a catalyst whose gas purifying efficiency is least affected by a high temperature of exhaust gases, it would still be necessary despite its disadvantages to keep relying on the system either supported with a reheating device, or installed closer to an engine.
Further, heretofore, a catalytic converter for purifying exhaust gases installed in an exhaust passage has, as referred to earlier, been generally designed to enhance its purifying efficiency utilizing a high temperature of exhaust gases. The high temperature contributes, however, to increasing the emission resistance of exhaust gases, thus falling off an automotive engine power and lowering a gas mileage.
And, heretofore, platinum, palladium and rhodium have remained the principal components of catalysts used for a catalytic converter for purifying gases. Generally, more than 2 grams of such a noble metal are required for a 2-liter automobile. It is vitally important, required for a 2-liter automobile. It is vitally important, therefore, that given its limited supply a prosperity of related industries should be sustained by developing a catalyst comprising noble metals of less than 0.2 grams per an automobile, or one comprising alternative catalytic components.